1. Field of the Invention
The present invention relates to a radio communication terminal and a transmission power control method, and more particularly to a radio communication terminal and a transmission power control method for a Wideband Code Division Multiple Access (hereinafter referred to as W-CDMA).
2. Description of the Related Art
In recent years, W-CDMA technologies have been developed as third-generation mobile communication means, and put into service. InW-CDMA, a cellular phone capable of performing power control includes an inner loop portion and an outer loop portion. The inner loop portion makes a request to a base station from which the cellular phone directly receives radio waves for an increase or a decrease in electric power at short time intervals (e.g., every 666 μs). The outer loop portion determines an appropriate power control reference value (i.e., a target signal to interference power ratio (hereinafter referred to as SIR)) based on a target quality value (e.g., an error rate) for predetermined long period (e.g., every several hundred milliseconds to several seconds). Such known art is disclosed in, for example, “Yasashii IMT-2000 (IMT-2000 Made Easy)” by Kota Kinoshita, Denki Tsuushin Kyokai (The Telecommunications Association), 2001, pp. 42-47.
FIG. 1 illustrates an exemplary circuit for conducting transmission power control (hereinafter referred to as TPC) in a known cellular phone. In an inner loop portion, a signal received by a radio transmitting/receiving section 100 is de-spread by a de-spreader 101, and then RAKE-combined by a RAKE receiving section 102. An SIR measuring section 107 measures a received SIR with regard to the RAKE-combined signal. An SIR comparing section 108 compares the received SIR with a target SIR. Based on the result of the comparison, a TPC bit generating section 109 outputs transmission power control information to the radio transmitting/receiving section 100. The information is transmitted to a base station. When the received SIR is lower than the target SIR, the inner loop portion transmits to the base station a request for an increase in the transmission power, while when the received SIR is equal to or higher than the target SIR, the inner loop portion transmits to the base station a request for a decrease in the transmission power.
In an outer loop portion, a long-time-interval quality measuring section 103 measures the communication quality on received data every time the received data reaches a predetermined amount. A quality comparing section 105 compares the measured communication quality with a target communication quality retained by a target quality information retaining section 104, sets a target SIR based on the result of the comparison, and transmits the target SIR to a target SIR retaining section 106. Alternatively, the target SIR retaining section 106 may set the target SIR based on the result of the comparison. When the measured communication quality is lower than the target communication quality (e.g., error rate of 1%) (i.e., the error rate is higher than 1%), the outer loop portion increases the target SIR by a predetermined amount, while when the measured communication quality is equal to or higher than the target communication quality, the outer loop portion decreases the target SIR by a predetermined amount.
Usually, a transmitting side divides a transmission information bit string per unit block, with an error check bit being added to each block. A receiving side receives a predetermined number of blocks, and calculates a block error rate (hereinafter referred to as BLER), i.e., a rate of blocks including an error. The BLER is included in the above-mentioned communication quality. For example, in packet communications, only a very small amount of data is received. This may require a long time to receive the predetermined number of blocks and to determine the BLER. Therefore, a high target SIR is maintained and a higher transmission power continues to be requested to the base station for the long time. Such a higher transmission power may result in increased interference with other users.